Conventional steam turbines have two types of steam valves. One is a main steam valve (also referred to as “main stop valve”). The other is a governing valve.
Both of the valves are provided at an upstream side of the steam turbine. The function of the main steam valve is to shut down steam flow to the turbine. The governing valve, which is provided at a downstream side of the main steam valve, controls the flow rate of steam introduced into the turbine during the operation.
When starting up a steam turbine for a power plant, such as a thermal power plant or a nuclear power plant, a warming up operation is usually necessary to prevent turbine components from being subjected to high stress due to sudden exposure to the high temperature steam generated at the steam generator. This is also true for the governing valve.
When warming up the governing valve during the warming up operation, which is typically until an initial load operation, the main steam valve controls the flow rate of the steam. For this reason, some conventional steam turbines have a main steam valve that also functions to control the flow rate of steam. This function is accomplished by installing a bypass valve inside of the main steam valve.
FIG. 8 is a schematic sectional view showing a conventional main steam valve having a bypass valve inside. In FIG. 8, a valve chamber 1 contains a main valve body 2, and a bypass valve body 3. The main valve seat 4 cooperates with the main valve body 2. A bypass steam flow 5 occurs when bypass valve rod 6 moves downwardly to open the connected bypass valve body 3.
As shown in FIG. 8, bypass valve body 3 is installed in the main valve body 2. During the warming up operation, main valve body 2 fits in the main valve seat 4 and shuts off the steam flow between the main valve body 2 and main valve seat 4. The main valve has a hollow recess in its inside, and the bypass steam flow 5 goes through this recess. The bypass valve body 3 is provided inside the hollow recess. The inner surface of the main valve body 2 serves as a valve seat for the bypass valve. Therefore, the flow rate of the steam flow 5 can be controlled by moving the bypass valve rod 6 up or down along its axial direction.
However, when controlling the steam flow rate by the bypass valve during the warming up operation, since the flow rate necessary may be relatively small, the flow velocity of the steam flow 5 that passes the bypass valve may be very high. The high velocity steam passing through the bypass valve can cause erosion on the bypass valve body 3 or on the bypass valve rod 6, due to a very small amount of oxidants or drains (waterdrops) which may be included in the steam flow 5. When this erosion becomes too severe, it may cause breakage of the bypass valve body 3 or bypass valve rod 6.
Conventional main steam valves, which are improved to deal with these problems, are described in Japanese Patent Publication (Kokai) No. 57-151006. FIGS. 9 and 10 are schematic sectional views showing this type of modified conventional main steam valve.
As shown in FIGS. 9 and 10, the main valve comprises a main valve body 2 and a main valve seat 4. The main valve body 2 has a substantially cylindrical shape. The main valve seat 4 is coaxially aligned with the main valve body 2, so that the outer surface of the main valve body 2, particularly at its bottom portion, can fit into the valve seat 4. In FIG. 9, the outer side of the main valve seat 4 (substantially upper portion of the main valve seat 4) is the upstream side of the flow, and the inner side of the main valve seat 4 (lower portion of the main valve seat 4) is the downstream side.
The bypass valve body 3 is installed in the main valve body 2, e.g., coaxially in the main valve body 2. A bypass valve rod 6 is connected to the bypass valve body 3 for moving the bypass valve body 3. Hence, The bypass valve body 3, provided in the main valve body, is movable in the axial direction.
The bypass valve body 3 penetrates through the main valve body 2. In other words, the bypass valve body 3 projects above the main valve body 2 toward the upstream side. A side surface 7 of the bypass valve body 3 fits into the inner surface of the main valve body 2. Bypass valve body 3 has a hollow interior, which forms a steam passageway 9 coaxially inside of the bypass valve body 3. The upper side of the steam passageway 9 is closed, while the lower side is open and forms steam outlet holes 10, which introduce the steam from the steam passageway 9 to the downstream side.
The upper portion of the side surface 7 has a steam inlet 8. The steam inlet 8 comprises a plurality of inlet rows arranged along the side surface 7, with each of the inlet rows comprising a plurality of openings. With this configuration, even though the main valve is shut, steam flow 5 can be introduced into the steam passageway 9 through the steam inlet 8, and then to the downstream side via steam holes 10. FIG. 9 shows the state of the bypass valve when it is fully opened. When it is closed by moving valve rod downwardly, the inlet openings are contained within the main valve body 2 and are thereby closed.
The steam introduced from the steam inlet 8 goes toward the center of the steam passageway 9. When passing through the steam inlet 8, the steam has a high velocity; however, the kinetic energy of the steam is weakened around the center portion of the steam passageway since the steam flow 5 is entering from all sides around the side surface 7. Therefore, the steam in the steam passageway 9 has relatively low velocity in the horizontal direction. Then the steam flow 5 moves downwardly in the steam passageway 9, with a recovery of its pressure, and goes to the downstream side through the steam holes 10.
The flow rate of the steam introduced into the steam passageway 9 can be controlled by adjusting an axial position of the bypass valve body 3. When the bypass valve rod 6 moves downwardly along an arrow X, the side surface 7 is drawn into the main valve body 2. Thus, an area of the steam inlet 8 above the main valve body 2, which is the sum of sizes of the openings of the steam inlet 8 above the main valve body 2, decreases, and the flow rate of the steam also decreases.
On the other hand, when the bypass valve rod 6 moves upwardly, the side surface 7 is pulled out from inside of the main valve body 2. Hence, the area of the steam inlet 8 above the main valve body 2, which is the sum of sizes of the openings of the steam inlet 8 above the main valve body 2, increases, and the flow rate increases.
In FIG. 10, which is a sectional view taken along the line X-X shown in FIG. 9, a keyway 11 is provided on the lower portion of the side surface 7 of the bypass valve. The keyway 11 comprises a groove. A key 12 which is coupled with the keyway is provided on the inner surface of the main valve body 2. The key 12 comprises a projection, fitting with the keyway, which is a groove. The keys 12 and the keyway 11, coupled together, prevent the bypass valve body 3 from rotating around its axis.
FIG. 7 is a schematic graph showing comparison of flow characteristics of various bypass valves. The horizontal axis indicates the degree of valve lift, and the vertical axis indicates the flow rate of the steam. The conventional bypass valve shown in FIG. 8 has the characteristic indicated as A in FIG. 7.
As described above, the flow rate of the conventional bypass valve shown in FIGS. 9 and 10 depends on the sum of the sizes of the openings of the steam inlet 8 positioned above the main valve body 2. Since the steam inlet 8 comprises a plurality of inlet rows, each of which comprises a plurality of openings, arranged along the side surface 7, a significant number of inlet rows arranged along the side surface in the axial direction are needed to ensure that the flow rate is large enough for the bypass valve to properly function. Thus, a larger stroke, which means a larger valve lift, is necessary for the bypass valve shown in FIGS. 9 and 10. This may make the motion of the bypass valve unstable. Further, the size of the main steam valve may need to be enlarged.
The characteristic of the bypass valve shown in FIGS. 9 and 10 is represented by the line B in FIG. 7. As shown in FIG. 7, from the flow characteristic point of view, the conventional bypass valve shown in FIGS. 9 and 10 is inferior to the conventional bypass valve shown in FIG. 8. Even if the conventional bypass valve shown in FIGS. 9 and 10 has twice of the valve lift of the conventional bypass valve shown in FIG. 8, the flow rate of the conventional bypass valve shown in FIGS. 9 and 10 is less than that of the conventional bypass valve shown in FIG. 8.
Since the size of the conventional main steam valve shown in FIGS. 9 and 10 is larger than that of the conventional main steam valve shown in FIG. 8, it may furthermore be difficult to replace the conventional main steam valve shown in FIG. 8 with the modified conventional main steam valve shown in FIGS. 9 and 10. Further, the velocity of the steam around the opening of the steam inlet 8 is still relatively high, and it may cause erosion.
Also, there is another problem with the modified conventional main steam valve that contains two sets of keys 12 and keyways 11. The use of two keys/keyways, which prevent the bypass valve from moving in an unexpected way, are not enough to perform that function. Especially when the bypass valve is fully opened, main valve body 2 vibrates due to the steam flow 5 and may result in wearing of the side surface 8 of the bypass valve and the inner surface of the main valve body 2. The inner surface of the main valve body 2 and the side surface 7 of the bypass valve body 3 have a cylindrical contact surface. Since the area of the cylindrical contact surface is relatively large, it is often a problem that debris becomes lodged in between the gap of the cylindrical contact surfaces. This may obstruct free movement of the surfaces.